Solubility and solution mechanism of H2O in alkali silicate melts and glasses at high pressure and temperature

The solubility behavior of H₂O in melts in the system Na₂O-SiO₂-H₂O was determined by locating the univariant phase boundary, melt = melt + vapor in the 0.8-2 GPa and 1000°-1300°C pressure and temperature range, respectively. The NBO/Si-range of the melts (0.25-1) was chosen to cover that of most natural magmatic liquids. The H₂O solubility in melts in the system Na₂O-SiO₂-H₂O (X_H2O) ranges between 18 and 45 mol% (O = 1) with (∂X_H2O/∂P)_T∼14-18 mol% H₂O/GPa. The (∂X_H2O/∂P)_T is negatively correlated with NBO/Si (= Na/Si) of the melt. The (∂X_H2O/∂T)_P is in the −0.03 to +0.05 mol% H₂O/°C range, and is negatively correlated with NBO/Si. The [∂X_H2O/∂(NBO/Si)]_P,T is in the −3 to −8 mol% H₂O/(NBO/Si) range. Melts with NBO/Si similar to basaltic liquids (∼0.6-∼1.0) show (∂X_H2O/∂T)_P<0, whereas more polymerized melts exhibit (∂X_H2O/∂T)_P>0. Complete miscibility between hydrous melt and aqueous fluid occurs in the 0.8-2 GPa pressure range for melts with NBO/Si ≤0.5 at T >1100°C. Miscibility occurs at lower pressure the more polymerized the melt.     

Solution behavior of reduced COH volatiles in silicate melts at high pressure and temperature

The solubility and solution mechanisms of reduced COH volatiles in Na₂OSiO₂ melts in equilibrium with a (H₂ + CH₄) fluid at the hydrogen fugacity defined by the iron-wüstite + H₂O buffer [f_H2(IW)] have been determined as a function of pressure (1-2.5 GPa) and silicate melt polymerization (NBO/Si: nonbridging oxygen per silicon) at 1400 °C. The solubility, calculated as CH₄, increases from ∼0.2 wt% to ∼0.5 wt% in the melt NBO/Si-range ∼0.4 to ∼1.0. The solubility is not significantly pressure-dependent, probably because f_H2(IW) in the 1-2.5 GPa range does not vary greatly with pressure. Carbon isotope fractionation between methane-saturated melts and (H₂ + CH₄) fluid varied by ∼14‰ in the NBO/Si-range of these melts.The (C..H) and (O..H) speciation in the quenched melts was determined with Raman and ¹H MAS NMR spectroscopy. The dominant (C..H)-bearing complexes are molecular methane, CH₄, and a complex or functional group that includes entities with CCH bonding. Minor abundance of complexes that include SiOCH₃ bonding is tentatively identified in some melts. There is no spectroscopic evidence for SiC or SiCH₃. Raman spectra indicate silicate melt depolymerization (increasing NBO/Si). The [CH₄/CCH]^melt abundance ratio is positively correlated with NBO/Si, which is interpreted to suggest that the (CCH)-containing structural entity is bonded to the silicate melt network structure via its nonbridging oxygen. The ∼14‰ carbon isotope fractionation change between fluid and melt is because of the speciation changes of carbon in the melt.     

Barium partitioning between alkali feldspar and silicate liquid at high temperature and pressure

Barium partitioning between alkali feldspar and a natural trachyte liquid, enriched with barium, has been determined as a function of pressure and temperature from 10 to 25 kb and 900°–1100° C. Both long duration experiments and a re-equilibration experiment suggest close approach to equilibrium. Partition coefficients (D _Ba) decrease as both temperature and pressure increase (e.g., D _Ba changes from 8.71 at 10 kb, 900° C to 1.48 at 25 kb, 1100° C). Water activity also controls the barium partitioning with a marked decrease in D _Ba ^af/liq for addition of less than 0.8 wt% H₂O, but with no apparent additional effect for higher water contents in the bulk composition (e.g., from 0.8–4.2 wt% H₂O). The composition of alkali feldspar also has a significant effect on D _Ba ^af/liq , but the data obtained do not allow derivation of a complete D-Or relationship. These new data suggest that Henry's Law is obeyed for most of the barium concentrations examined, and the limit of Henry's Law behaviour for barium in alkali feldspar is as high as 6 wt% BaO in alkali feldspar and 1.2 wt% BaO in the melt, similar to the results of Long (1978). The experimental results broadly overlap with natural data for D _Ba, determined from coexisting alkali feldspar phenocrysts and glass (or groundmass).     

Solution mechanisms of silicate in aqueous fluid and H2O in coexisting silicate melts determined in-situ at high pressure and high temperature

The structure of H₂O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na₂O·4SiO₂–H₂O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H₂O at nominal densities, ρ_fluid, (from EOS of pure H₂O) of 0.90 and 0.78 g/cm³. With ρ_fluid = 0.78 g/cm³, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρ_fluid = 0.90 g/cm³, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na₂O·4SiO₂ starting material.Molecular H₂O (H₂O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H₂O)-ratio is positively correlated with temperature. The OH/(H₂O)° in melts is greater than in coexisting fluids. Structural units of Q³, Q², Q¹, and Q⁰ type are observed in all phases under all conditions. An expression of the form, 12Q³ + 13H₂O2Q² + 6Q¹ + 4Q⁰, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.     

The Partitioning of Tungsten bwtween Aqueous Fluids and Silicate Melts

An experimental study has been carried out to determine the partition coefficients of tungsten between aqueous fluids and granitic melts at 800 °C and 1.5 kb with natural granite as the starting material. The effects of the solutions on the partition coefficients of tungsten show a sequence of P > CO ₃ ²⁻ > B > H₂O. The effects are limited (generallyK _D < 0.3) and the tungsten shows a preferential trend toward the melt over the aqueous fluid. The value ofK _D increases with increasing concentration of phosphorus; theK _D increases first and then reduces with the concentration of CO ₃ ²⁻ when temperature decreases, theK _D between the solution of CO ₃ ²⁻ and the silicate melt increases, and that between the solution of B₄O ₇ ²⁻ and the silicate melt decreases. The partition coefficients of phosphorus and sodium between fluids and silicate melts have been calculated from the concentrations of the elements in the melts. TheK _D value for phosphorus is 0.38 and that for sodium is 0.56. Evidence shows that the elements tend to become richer and richer in the melts.     

Dual speciation of nitrogen in silicate melts at high pressure and temperature: An experimental study

Solubility and speciation of nitrogen in silicate melts have been investigated between 1400 and 1700 °C and at pressures ranging from 10 to 30 kbar for six different binary alkali and alkaline-earth silicate liquids and a Ca-Mg-alumino silicate. Experiments were performed in a piston-cylinder apparatus. The nitrogen source is silver azide, which breaks down to Ag and molecular N₂ below 300 °C. At high pressure and temperature, the nitrogen content may be as high as 0.7 wt% depending on the melt composition, pressure, and temperature. It increases with T, P and the polymerization state of the liquid. Characterization by Raman spectroscopy and ¹⁵N solid state MAS NMR indicates that nitrogen is not only physically dissolved as N₂ within the melt structure like noble gases, but a fraction of nitrogen interacts strongly with the silicate network. The most likely nitrogen-bearing species that can account for Raman and NMR results is nitrosyl group. Solubility data follow an apparent Henry’s law behavior and are in good agreement with previous studies when the nitrosyl content is low. On the other hand, a significant departure from a Henry’s law behavior is observed for highly depolymerized melts, which contain more nitrosyl than polymerized melts. Possible solubility mechanisms are also discussed. Finally, a multi-variant empirical relation is given to predict the relative content of nitrosyl and molecular nitrogen as a function of P, T, and melt composition and structure. This complex speciation of nitrogen in melts under high pressure may have significant implication concerning crystal-melt partitioning of nitrogen as well as for potential elemental and isotopic fractionation of nitrogen in the deep Earth.     

The partitioning of copper and molybdenum between silicate melts and aqueous fluids

The partitioning of copper and molybdenum between silicate melts and aqueous fluids has been determined at 750°C, and 1.4 Kb. The experiments were conducted in a $\text{1}\text{2}$ inch ID, rapid quench, cold seal pressure vessel. The aqueous and glass phase run products were analyzed by atomic absorption spectrophotometry and ion microprobe, respectively. The vapor/melt partition coefficient for copper, $\text{D}{}^{\mathrm{<mtext>v</mtext><mtext>l</mtext>}}{}_{\mathrm{Cu}}$, defined as the ratio of the concentrations of copper in the vapor to copper in the melt was found to be $\text{D}{}^{\mathrm{<mtext>v</mtext><mtext>l</mtext>}}{}_{\mathrm{Cu}}\text{= (9.1 ± 2.5)m}{}^{\mathrm{v}}{}_{\mathrm{Cl}}$ at NNO up to at least 4.5 moles of chlorine per kg of solution. The partition coefficient for molybdenum is equal to 2.5 ± 1.6 at NNO and QFM; its value is independent of the fluorine concentration of the melt up to at least 1.7 wt. percent fluorine, and of the chlorine concentration up to at least 4.5 moles of chlorine per kg of solution. Copper is probably present in the univalent state in both the silicate melt and in the associated aqueous phase at NNO; the most important aqueous complex of copper is probably CuCl⁰. Molybdenum is probably present in the aqueous phase as one or more molybdate species.     

The solution behavior of H2O in peralkaline aluminosilicate melts at high pressure with implications for properties of hydrous melts

Solubility and solution mechanisms of H₂O in depolymerized melts in the system Na₂O-Al₂O₃-SiO₂ were deduced from spectroscopic data of glasses quenched from melts at 1100 °C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na₂Si₄O₉-Na₂(NaAl)₄O₉] with Al/(Al+Si) = 0.00-0.25. The H₂O solubility was fitted to the expression, X_H2O=0.20+0.0020f_H2O-0.7X_Al+0.9(X_Al)², where X_H2O is the mole fraction of H₂O (calculated with O = 1), f_H2O the fugacity of H₂O, and X_Al = Al/(Al+Si). Partial molar volume of H₂O in the melts, , calculated from the H₂O-solulbility data assuming ideal mixing of melt-H₂O solutions, is 12.5 cm³/mol for Al-free melts and decreases linearly to 8.9 cm³/mol for melts with Al/(Al+Si) ∼ 0.25. However, if recent suggestion that is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H₂O, , increases with Al/(Al+Si).Solution mechanisms of H₂O were obtained by combining Raman and ²⁹Si NMR spectroscopic data. Degree of melt depolymerization, NBO/T, increases with H₂O content. The rate of NBO/T-change with H₂O is negatively correlated with H₂O and positively correlated with Al/(Al+Si). The main depolymerization reaction involves breakage of oxygen bridges in Q⁴-species to form Q² species. Steric hindrance appears to restrict bonding of H⁺ with nonbridging oxygen in Q³ species. The presence of Al³⁺ does not affect the water solution mechanisms significantly.     

Nature of polymerization and properties of silicate melts and glasses at high pressure

The structures of sodium silicate and aluminosilicate glasses quenched from melts at high pressure (6-10 GPa) with varying degrees of polymerization (fractions of nonbridging oxygen) were explored using solid-state NMR [¹⁷O and ²⁷Al triple-quantum magic-angle spinning (3QMAS) NMR]. The bond connectivity in melts among four and highly coordinated network polyhedra, such as ^[4]Al, ^[5,6]Al, ^[4]Si, and ^[5,6]Si, at high pressure is shown to be significantly different from that at ambient pressure. In particular, in the silicate and aluminosilicate melts, the proportion of nonbridging oxygen (NBO) generally decreases with increasing pressure, leading to the formation of new oxygen clusters that include 5- and 6-coordinated Si and Al in addition to 4-coordinated Al and Si, such as ^[4]Si-O-^[5,6]Si, ^[4]Si-O-^[5,6]Al and Na-O-^[5,6]Si. While the fractions of ^[5,6]Al increase with pressure, the magnitude of this increase diminishes with increasing degrees of ambient-pressure polymerization under isobaric conditions. Incorporating the above structural information into models of melt properties reproduces the anomalous pressure-dependence of O²⁻ diffusivity and viscosity often observed in silicate melts.     

Solubility of platinum and palladium in silicate melts under high water pressure as a function of redox conditions

The solubility of platinum and palladium in a silicate melt of the composition Di ₅₅ An ₃₅ Ab ₁₀ was determined at 1200°C and 2 kbar pressure in the presence of H₂O-H₂ fluid at an oxygen fugacity ranging from the HM to WI buffer equilibria. The influence of sulfur on the solubility of platinum in fluid-bearing silicate melt was investigated at a sulfur fugacity controlled by the Pt-PtS equilibrium at 1200°C and a pressure defined in such a way that the \(f_{H_2 O} \) and \(f_{O_2 } \) values were identical to those of the experiments without sulfur. The experiments were conducted in a high pressure gas vessel with controlled hydrogen content in the fluid. Oxygen fugacity values above the NNO buffer were controlled by solid-phase buffer mixtures using the two-capsule technique. Under more reducing conditions, the contents of H₂O and H₂ were directly controlled by the argon to hydrogen ratio in a special chamber. The hydrogen fugacity varied from 5.2 × 10^?2 bar (HM buffer) to 1230 bar (\(X_{H_2 } \) = 0.5). Pt and Pd contents were measured in quenched glass samples by neutron activation analysis. The results of these investigations showed that the solubility of Pt and Pd increases significantly in the presence of water compared with experiments in dry systems. The content of Pd within the whole range of redox conditions and that of Pt at an oxygen fugacity between the HM to MW buffer reactions are weakly dependent on \(f_{O_2 } \) and controlled mainly by water fugacity. This suggests that, in addition to oxide Pt and Pd species soluble at the ppb level in haplobasaltic melts, much more soluble (ppm level) hydroxide complexes of these metals are formed under fluid-excess conditions. Despite a decrease in water fugacity under reducing conditions, Pt solubility increases sharply near the MW buffer. It was shown by electron paramagnetic resonance spectrometry that, in contrast to dry melts, fluid-saturated silicate melts do not contain a pure metal phase (micronuggets). Therefore, the increase in Pt solubility under reducing conditions can be explained by the formation of Pt hydride complexes or Pt-fluid-silicate clusters. At a sulfur fugacity controlled by the Pt-PtS equilibrium, the solubility of Pt in iron-free silicate melts as a function of redox conditions is almost identical to that obtained in the experiments without sulfur at the same water and oxygen fugacity values. These observations also support Pt dissolution in iron-free silicate melts as hydroxide species.     

Melt inclusions in pegmatite quartz: complete miscibility between silicate melts and hydrous fluids at low pressure

Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 °C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich H₂O-poor melt and a silicate-poor H₂O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) − water system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 °C in steps of 50 °C by conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H₂O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions define a solvus boundary in an X_H2O–T pseudobinary system. At 500 °C, the silicate-rich melt contains about 2.5 wt% H₂O, and the conjugate water-rich melt about 47 wt% H₂O. The solvus closes rapidly with increasing temperature. At 650 °C, the water contents are about 10 and 32 wt%, respectively. Complete miscibility is attained at the critical point: 712 °C and 21.5 wt% H₂O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid–melt systems. At shallow intrusion levels, at T ≥ 720 °C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to “normal” silicate melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable to an aqueous fluid. Received: 15 September 1999 / Accepted: 10 December 1999     

Solubility of Au in Cl- and S-bearing hydrous silicate melts

The solubility of Au in Cl- and S-bearing hydrous rhyodacitic and andesitic melts has been experimentally investigated at 1050 °C, 200 MPa and log fO₂ close to the Ni/NiO solid oxygen buffer (NNO). The concentrations of Au in the experimental glasses have been determined using Laser Ablation ICP-MS (LA) with special efforts to avoid incorporation of Au micronuggets in the analysis. It is concluded that metal micronuggets are an experimental artefact and produced by Au partitioning into the fluids during heating with consequent precipitation on fluid dissolution in the melting glass powder. Hence, the micronuggets do not represent quench phases and must be excluded from the analysis. The micro-analytical data obtained by LA show that Au concentrations vary from ∼0.2 to ∼2.5 ppm by weight, generally consistent with the literature data for other melt compositions. The measured Au concentrations increase with increasing amounts of Cl and S dissolved in the silicate melt and show a correlation with the apparent activities of Cl and S in the system. The apparent activities of Cl and S are defined by the simplified linear relationship between volatile concentrations in the melt and activity of volatiles. The maximum activity (a^∗ = 1) is assumed to be reached at the saturation of the systems in respect of Cl-rich brine or FeS liquid for Cl and S, respectively. The dependence of Au solubility on the concentrations/activities of Cl and S at the fixed redox conditions shows that Au may form not only oxide- but also Cl- and S-bearing complexes in silicate melts. Furthermore, it indicates that exsolution of S and Cl from the melt by degassing/segregation/crystallization processes may lead to mobilization and extraction of Au into the fluid, liquid and/or mineral phase(s).     

Density of peridotite melts at high pressure

Densities of ultramafic melts were determined up to 22 GPa by relative buoyancy experiments. Olivine and diamond were used as buoyancy markers. We confirmed that the density crossover of PHN 1611 melt and its equilibrium olivine (Fo94) occurs at around 13.5 GPa and 2030 °C and that olivine floats from deeper regions in the magma ocean of the primordial terrestrial mantle. The comparison of the compression curves of basic and ultrabasic melts implies that the basic melt is more compressible. This can be explained by the difference in the amount of compressible linkage of SiO_n and AlO_n polyhedra. The interstitial melt trapped by the density crossover can be the cause of the impedance anomaly of the seismic wave in the deep upper mantle.     

CO2 solubility and solubility mechanisms in silicate melts at high pressures

CO₂ solubility has a slight negative temperature dependence in olivine melilitite at 30 kb with 9% CO₂ dissolved at 1,450 °C, 8.5% at 1,550 °C and 8.3% at 1,650° C. CO₂ is dissolved as the carbonate molecule (CO ₃ ^2– ) only. Feldspar melts (albite-anorthite) dissolve much less CO₂ at 30 kb (around 2%) with a slight increase with increasing anorthite content. A CO₂ absorption peak in infrared spectra of albite-rich glasses diappears in favour of the CO ₃ ^2– peak with increasing anorthite content. It is inferred that CO₂ was present as CO ₃ ^2– in albite-rich melts also, but reverts to CO₂ during quenching because of bonding differences related to Ca²⁺ and Na⁺ in the melts.     

高温硅酸盐熔体粘度与网络分数维值的相关性研究

对高温硅酸盐熔体粘度的估算一直是国际地学界热点问题之一,本文在研究了高温硅酸盐熔体网络分数维值的基础上,建立了估算熔体粘度的新模式 (简称 FD模式 ) ,阐明了熔体粘度值除了与温度成反比外,还与分子网络介观尺度的自相似比 ri 和分数维值 D相关,即与分子网络 (纳米尺度以上 )中的桥氧数 Ni成正比 (Ni∝ ) , 与单位硅氧四面体中的非桥氧数成反比.经对 4个硅酸盐系列高温熔体的粘度测定证实, FD模式的理论计算值与实测值吻合,且优于现今国际通用的 VTF模式.为探索岩浆迁移演化规律和完善新型低维材料的性能提供了新的理论依据.     

Kinetics and mechanisms of iron redox reactions in silicate melts: The effects of temperature and alkali cations

The kinetics and the mechanisms of iron redox reactions in molten Fe-bearing pyroxene compositions have been investigated by Raman spectroscopy and X-ray absorption Near Edge Structure (XANES) experiments at the iron K-edge. The former experiments have been made only near the glass transition whereas the latter have also been performed from about 1300 to 2100 K. The same kinetics are observed with both techniques. They are described by characteristic times that depend primarily on temperature and not on the initial redox state. At high temperatures, where both kinds of reactions could be investigated, these times are similar for oxidation and reduction. From these characteristic times we have calculated as a function of temperature and composition a parameter termed effective redox diffusivity. For a given melt, the diffusivities follow two distinct Arrhenius laws, which indicate that the mechanisms of the redox reaction are not the same near the glass transition and at high temperatures. As is now well established, diffusion of divalent cations is the dominant mechanism at low temperatures but the enhanced kinetics observed for alkali-bearing melts indicate that Li⁺ and Na⁺ also participate in ionic transport. At superliquidus temperatures, in contrast, diffusion of oxygen represents the dominant mechanism.     

Diamond stability in silicate-halogenide melts at high pressure

Chlorides of alkali metals of mantle origin and with the possibility to influence the preservation of natural diamonds were found in kimberlites of deep horizons of Udachnaya pipe (Yakutia).     

Viscosity of silicate melts in CaMgSi2O6–NaAlSi2O6 system at high pressure

In situ X-ray viscometry of the silicate melts was carried out at high pressure and at high temperature. The viscosity of the silicate melts in the diopside(Di)–jadeite(Jd) system was determined in the pressure range from 1.88 GPa to 7.9 GPa and in the temperature range from 2,003 K to 2,173 K. The viscosity of the Di 25%–Jd 75% melt decreases continuously to 5.0 GPa, whereas the viscosity of the Di 50%–Jd 50% melt increases over 3.5 GPa. The viscosity of the Di50%–Jd 50% melt reaches a minimum around 3.5 GPa. Since the amounts of silicon in the two melts are the same, the difference in the pressure dependence of the viscosity may be controlled by another network-forming element, i.e., aluminum. The difference in the pressure dependence of the viscosities in the melts with two intermediate compositions in the Di–Jd system is estimated to be due to the difference in the melt structures at high pressures and high temperatures.